1. Field of the Invention
The present invention relates to a quartz optical waveguide which can reduce transmission loss of guided light and a method for producing the same.
2. Description of the Related Art
Among optical waveguides, a quartz optical waveguide comprising quartz glass attracts attentions since it has a low light transmission loss and can be connected with a quartz optical fiber with a low connection loss.
In general, such quartz optical waveguide is produced by a combined method of glass film formation by a flame hydrolysis deposition (FHD) and fine processing of the formed glass film by reactive ion etching (RIE) (cf. Masao Kawachi, "Quartz Optical Waveguides and Their Application in Integrated Optical Elements", OPTICS, 18 (12), December 1989, 681-686).
The above method for producing a quartz optical waveguide will be explained by making reference to FIG. 1.
As shown in FIG. 1A, a glass-forming raw material such as SiCl.sub.4, TiCl.sub.4 and the like are supplied to a burner 2 together with a fuel gas (e.g. hydrogen gas, oxygen gas, etc.) and hydrolyzed and oxidized in an oxyhydrogen flame 2 to form fine particles 3 (soot) of glass. The glass soot is then deposited on a substrate 4 such as a silicon wafer to successively form films of glass soot 5a and 5b which have different compositions from each other. The deposited glass films on the substrate 4 are vitrified by heating them at a high temperature to obtain a buffering layer 6a and a core layer 6b as shown in FIG. 1B.
The above method is FHD.
Then, by RIE, unnecessary parts of the core layer 6b are removed to remain a ridge-form core part 6c as shown in FIG. 1C. Again, by FHD, a cladding layer 6d is formed to surround the core part 6c to form an embedded type quartz optical waveguide 7 as shown in FIG. 1D.
Though the light transmission loss through the quartz optical waveguide has been reduced to about 0.1 dB/cm, its further decrease is desired since a light transmission loss through a quartz optical fiber has been reduced to 1 dB/km.
As quality of a device comprising optical waveguide has been much improved, a longer waveguide length is required. Then, it is an important object to further decrease the light transmission loss of the quartz optical waveguide.
The light transmission loss through the optical waveguide may be attributed to light scattering caused by irregular structures such as irregularities at an interface between a core part and a low refractive index part which surrounds the core part such as a cladding part or a buffering part.
The reason why the optical fiber has a much lower light transmission loss may be that, in the fabrication of the optical fiber, since a glass preform is once produced and it is drawn to a fiber having a diameter of, for example 125 .mu.m, irregularities at an interface between the core part and the cladding or buffering part are smoothened during drawing so that such irregularities have no substantial influence on the light transmission loss in the fabricated optical fiber.
On the other hand, when the quartz optical waveguide is produced by the method which has been explained by making reference to FIG. 1, the irregular structures which are formed during the production of the optical waveguide remain. In particular, when the unnecessary parts of the core layer are removed in the RIE step, it is very difficult to make the side walls of the core sufficiently smooth and to make the width of the core uniform in the longitudinal direction.
FIG. 2 schematically shows refractive index profiles in the common optical waveguide, and the refractive index discontinuously changes at interfaces between the core part and the cladding layer and between the core part and the buffering layer, that is, the refractive index has a so-called step profile. Therefore, when the optical waveguide is connected with an optical element having a different mode field such as the optical fiber, the connection loss increases.